This invention relates to an ultrasonic wave transmitting and receiving apparatus which is constructed such that an auxiliary signal is produced from a received signal of a limited quantity for the purpose of displaying an image having an excellent quality.
An ultrasonic wave tomography is used widely as a means not only for the diagnosis of a patient but also for examining the internal state of a body of materials, for example electrical materials. Thus, for example, an electro-acoustic transducing element 2 is contacted against the surface of a body 1 to be examined for transmitting an ultrasonic wave pulse into the body 1, as shown in FIG. 1. The pulse reflected by a heterogeneous portion 3, such as a defect or a deceased portion, is received by the electro-acoustic transducing element 2. This operation is repeated successively for various portions of the body 1, and the received signals are applied to a cathode ray tube for producing a tomograph of the heterogeneous portion on the screen of the cathode ray tube.
More particularly, the interval .tau..sub.1 between the transmission of the ultrasonic wave pulse and the receipt of the reflected pulse by the electro-acoustic transducing element 2 is proportional to the distance x.sub.1 between the element 2 and the heterogeneous portion 3 so that the interval .tau..sub.1 can be expressed by an equation .tau..sub.1 =2x.sub.1 /c, where c represents the sound velocity in the body 1. Accordingly, when an electron beam of a cathode ray oscilloscope is swept at a definite speed along the abscissa, and when the reflected pulse is subjected to a brightness modulation, the points corresponding to the fore and rear edges of the heterogeneous portion 3 will be lighted. Then, when the ultrasonic wave beam is shifted in a direction perpendicular to the direction of the beam and when the electron beam of the cathode ray tube is moved in the vertical direction corresponding to the scanning of the body 1 by the ultrasonic wave beam a tomographic image 5 of the heterogeneous portion 3 will be produced or the screen 4 of the cathode ray tube, thus giving an information regarding the state of the inside of the body 1.
The scanning of the body 1 by the ultrasonic wave beam can be accomplished by a linear scanning method wherein the electro-acoustic transducing element 2 is shifted a definite distance in a direction perpendicular to the beam, or a sector scanning method in which the element 2 is rotated in a sector or by a combination or modification of these methods. With a low scanning speed, since the display system of the conventional cathode ray tube has no memory function, it is not only impossible to directly observe the displayed tomographic image 5 but also to observe moving portions.
Accordingly, in order to decrease the flicker or to enable to observe an object which is moving at a high speed, it is necessary to sufficiently increase the scanning speed.
However, the sound velocity of the ultrasonic wave is not so high so that when the ultrasonic wave beam is transmitted in a given direction and then the beam is transmitted in a different direction after the reflected beam has completely disappeared the repetition period of the ultrasonic wave pulse would be limited thereby making it impossible to use a high speed scanning. Where the period of the ultrasonic pulse is given, the number of scannings per one second is determined. Accordingly, the spacing between the scanning lines or the screen is determined for a given field of view, that is the area of the section to be examined, and a given number of the frames per second. This factor determines the coarseness of the image. Denoting the area of the field by S, the number of the frames per second by N, the spacing between the scanning lines by P and the sound velocity by c, we obtain an equation P = 2mSN/c. Assuming that c = 1500 m/sec., the relationship between S, N and P will be shown by graphs shown in FIG. 3. In the above equation, m is a coefficient showing that the repetition period of the ultrasonic wave pulse is selected to be longer than the time of arrival of the reflected pulse in the field so as to prevent signals caused by the reflected pulse from a portion more remote from the electro-acoustic transducing element than the field of view, or by multiple reflections from appearing on the scanning lines for displaying an image. In the example shown in FIG. 3, it is assumed that m = 2. In other words, in the case of FIG. 3, the transmission of the next ultrasonic wave pulse is delayed by an interval equal to the sum of the interval between the transmission of the ultrasonic wave pulse and the receipt by a pulse reflected from a portion in a region more remote than the field of view, and a time equal to said interval. Usually, m is set about 1. Let us consider an interval in which a signal is displayed in the field of the screen as an effective time, and the time until the next ultrasonic wave pulse is transmitted as an idle time. Then, when the idle time is short, there is a fear that the signal caused by multiple reflection or by the pulse reflected by a remote portion will appear on the scanning line thus forming a false image. On the contrary, too long idle time (m is large) decreases the density of the scanning lines (or the field area or the number of frames) thus making it impossible to observe moving objects. This makes impossible to display with an actual time. Furthermore, where it is desired to simultaneously display a UCG as will be described later, the spacing between the scanning lines of the UCG would be decreased owing to a small number of samplings of the UCG.